At present, methods and devices based on ultrasound and density measuring are mainly used for measuring the gas content, and particularly the air content, of a fluid or liquid. In ultrasound measuring, the ultrasound is transmitted through the fluid to be measured and its attenuation is measured. The attenuation of the ultrasound is a function of the gas content in the fluid, so the more there is gas in the fluid, the more the ultrasound attenuates. In paper industry, the gas content of papermaking pulp or stock is typically measured by means of ultrasound measuring. The quality of the end product, i.e. paper, depends on the quality of the fluid papermaking pulp which is partly defined by its gas content.
Finnish Patent 84,299 discloses a solution, in which the air content of a suspension is determined by measuring the water content of the suspension at two different known pressures. U.S. Pat. No. 4,852,395 also discloses a solution, in which the gas content of a flowing liquid is determined by measuring the transmissivity of microwave radiation through the liquid at two different known pressures.
One problem with the gas content measuring based on the ultrasound attenuation is, however, that the method cannot be applied to suspensions containing considerably solid matter. For instance, the gas content measuring of pulp used for paper making can be done only, if the pulp consistency is less than 2%. Moreover, it is difficult to apply the method for installations of in-line type. This is why the ultrasound measuring devices have generally been developed for measuring a sample flow corresponding as closely as possible to pulp. On the other hand, one problem with the density measuring is that the measurement is also very sensitive to other components than air, which cause changes in concentration. Air is the most common gas in papermaking pulp. In papermaking pulp, typical density values of different suspension components are the following: water=1, air=0, wood fiber=1.3 to 1.5, and fillers=2.5 to 4.5. These figures show that even a slight decrease in filler amounts in pulp gives the impression that the air content of pulp has inreased.
The problem with microwave measurings carried out at two known pressures is that pressures employed have to be measured exactly at the point where the microwave measuring is also carried out. The pressure indicator is to be accurately calibrated to avoid systematic measuring errors in gas content.